There are two good articles on EJB3.0 and the Java Persistence API (JPA) this month:
“Writing Performant EJB Beans in the Java EE 5 Platform (EJB 3.0) Using Annotations” on the java.sun.com site, which provides performance comparisons between different annotation properties and describes what effect they have on performance. This is interesting information as it allows you to make an informed decision about how to define and configure your Entities in terms of performance.
OnJava.com has an introductory article on the JPA, “Standardizing Java Persistence with the EJB3 Java Persistence API” which walks you through from the begining – what is a Domain Model, what are Entities, what are annotations, what is the EntityManager etc. This is a good introductory article to get you started if you have not yet looked into EJB3.0